Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. In a typical leak test, a vacuum is applied to the fuel system. The integrity of the system is determined by monitoring the decay of the applied vacuum or by comparing the resulting fuel system pressure to an expected pressure. The vacuum source may be the intake manifold of the vehicle engine.
In some vehicles, such as hybrid electric vehicles, the vehicle engine may not run frequently, or may not generate enough vacuum to conduct a leak test. Such vehicles are required to have an evaporative leak check module (ELCM) coupled to the fuel system. The ELCM includes a vacuum pump that can be coupled to the fuel system for leak testing. When applying a vacuum to the fuel tank, fuel vapors may be drawn into the fuel vapor canister. Again, the limited engine run time may limit opportunities to purge the fuel vapor canister to intake. In particular, if the leak check is performed following a key-off event, the loaded fuel vapor canister may soak over a diurnal cycle, increasing bleed emissions.
A typical ELCM also contains a reference orifice. As a reference check, the ELCM may be isolated from the fuel system, and the vacuum pump activated to draw a vacuum on the reference orifice. The resulting pressure serves as a reference for leaks of equivalent size. By using an internal reference orifice, the ELCM automatically calibrates for factors such as ambient temperature, humidity, and barometric pressure. However, an ELCM coupled to the fuel tank via the fuel vapor canister will not compensate for fuel Reid Vapor pressure, as any fuel vapor will be adsorbed by the canister. In the case of highly volatile fuel, the fuel vapor may counteract the vacuum, and thus the fuel tank vacuum during the leak test may be unable to reach the reference vacuum during an allotted testing time. This may result in false failures of the ELCM based leak test.
The inventors herein have recognized the above problems and have developed systems and methods to at least partially address them. In one example, a method for a fuel system, comprising: indicating a leak in a fuel tank following applying a vacuum to the fuel tank by running a vacuum pump in a first direction; then purging a fuel vapor canister to the fuel tank by running the vacuum pump in a second direction, opposite the first direction. In this way, the fuel vapor canister may be purged of its contents following a fuel system leak check. This may reduce bleed emissions while increasing the efficiency of the vehicle, as the engine does not need to be turned on in order to purge the canister.
In another example, a fuel system for a vehicle, comprising: a fuel tank coupled to a fuel vapor canister via a fuel tank isolation valve; an evaporative leak check module coupled between the fuel vapor canister and atmosphere, the evaporative leak check module comprising a reversible vacuum pump; and a control system configured with instructions stored in non-transitory memory, that when executed cause the control system to: determine a reference vacuum threshold; then open a fuel tank isolation valve; determine a first fuel system pressure by drawing a vacuum on the fuel system via operating the reversible vacuum pump in a first direction; indicate a fuel system leak based on a comparison of the first fuel system pressure and the reference vacuum threshold; then purge contents of the fuel vapor canister to the fuel tank by operating the reversible vacuum pump in a second direction, opposite the first direction, while maintaining the fuel tank isolation valve open. In this way, the canister load may be reduced in hybrid vehicles, which may otherwise go for long periods without an opportunity to purge the canister to intake. Evaporative emissions regulations require testing of the fuel system, which may draw fuel vapor into the canister even if the engine is not in use. By incorporating a reversible vacuum pump, the canister can be cleaned, and fuel vapor returned to the fuel tank.
In yet another example, a fuel system for a vehicle, comprising: a fuel tank coupled to a fuel vapor canister via a fuel tank isolation valve; a canister vent valve coupled between the fuel vapor canister and atmosphere; an evaporative leak check module coupled between the fuel tank isolation valve and the fuel vapor canister, the evaporative leak check module comprising a reversible vacuum pump; and a control system configured with instructions stored in non-transitory memory, that when executed cause the control system to: open the fuel tank isolation valve; then determine a reference vacuum threshold; determine a first fuel tank pressure by drawing a vacuum on the fuel tank via operating the reversible vacuum pump in a first direction with the canister vent valve opened; determine a first fuel system pressure by drawing a vacuum on the fuel system via operating the reversible vacuum pump in a second direction, opposite the first direction, with the canister vent valve closed; indicate a fuel tank leak based on a comparison of the first fuel tank pressure to the reference vacuum threshold; and indicate a leak in the fuel system that is not in the fuel tank based on a comparison of the first fuel system pressure and the reference vacuum threshold. In this way, the reference vacuum threshold compensates for the fuel Reid Vapor Pressure. As such, when a vacuum is drawn on the fuel tank the reference vacuum threshold can be met during the allotted testing duration if no leak is present. This may reduce false failures and improve the accuracy of the leak test.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.